Hub-bearing units of the flanged type, for applications to drive wheels of motor vehicles, are known from the prior art. The document EP 2 602 123 A1, for example, describes a hub-bearing unit, which is asymmetrical in this case, for the wheel of a motor vehicle, comprising a flanged hub rotatable about an axis of rotation, a flange fixed to the flanged hub and placed transversely to the axis of rotation, a fixed ring positioned radially outside the flanged hub and provided with races axially spaced apart from each other, and two crowns of rolling bodies (such as balls) positioned between the fixed ring and the flanged hub. The flanged hub integrally forms a radially inner race for the axially outer crown of balls, while the radially inner race for the crown of axially inner balls is formed on an inner ring of the bearing, fitted radially and externally on to the flanged hub.
An embodiment of this type, especially when used in applications which are demanding in terms of transmitted loads, creates considerable local loads between the rings and the rolling bodies of the bearing; moreover, this embodiment cannot be used to produce a very strong or highly durable bearing.
Finally, it usually has large axial overall dimensions, due to the presence of the flange portion fixed to the flanged hub and transverse to the axis of rotation, and also because the axial lengths of the flanged bearing and the constant velocity joint will be added to one another when these components are mounted on the vehicle.
It should also be borne in mind that, while motor vehicle manufacturers require constant improvement in the performance of bearings, especially in terms of strength or at least rigidity, they also demand the provision of such performance in products at no extra cost, or even at lower cost. At the same time, motor vehicle manufacturers, aiming to comply with statutory requirements for continually decreasing CO2 emissions, are increasingly coming to view the weight of a vehicle as an essential parameter. The weight of all the components of the vehicle has to be reduced. An increase in the strength of a bearing will not usually be acceptable if it comes at the cost of a non-negligible increase in the weight of the component. It should also be borne in mind that many manufacturers no longer require individual components, but need pre-assembled modules, which are much simpler to handle for mounting on a car.
In order to improve the performance, and especially the strength of the bearing, the distance between the pressure centers must be increased. This may be done by increasing the diameter of the circumference of the centers of the rolling bodies (known as the pitch diameter) of the bearing. Such solutions are known and have been developed in order to improve performance to a substantial degree. Increasing the pitch diameter has the drawback that the volume, and therefore the weight, also increase dramatically, with the square of the value of the pitch diameter. This weight increase is usually unacceptable to motor vehicle manufacturers.
Another improvement can be made by providing a further increase in the diameter of the circumference of the centers of the rolling bodies, so that the constant velocity joint can be fitted into the bearing and the part known as the bell of the joint can be integrated with the hub, that is to say with the inner ring of the bearing. Clearly, the integration of the components enables both the weight and the cost of the whole unit to be reduced. The weight and cost can be reduced further by also integrating the small inner ring of the bearing, in the axially inner position, with the bell of the joint. In other words, the hub acts as a single inner ring of the bearing and as the bell of the constant velocity joint.
Clearly, these design guidelines are not sufficient in themselves for the development of a novel hub-bearing unit which has considerable strength while also being light, at no added cost.